1. Field of Invention
The invention relates to an electric power steering device, and a control method thereof. More particularly, the invention relates to an electric power steering device equipped with an electric actuator for assisting the driver's steering operation of a steering wheel, and a control method of the device.
2. Description of Related Art
A known electric power steering device of a vehicle detects the steering torque given to the steering wheel, and causes an electric motor to generate a steering assist torque in accordance with the detected steering torque. The thus-generated steering assist torque is controlled by a controller. The controller, for example, calculates a basic assist control amount (e.g., a basic assist torque) on the basis of the steering torque detected by the steering torque sensor, and the vehicle speed detected by a vehicle speed sensor. Furthermore, the controller calculates a compensation control amount (e.g., a compensation torque) for compensating for the basic assist control amount, and adds the compensation control amount to the basic assist control amount to calculate a final target assist control amount. Then, on the basis of the calculated target assist control amount, the controller controls the electrification of the electric motor so that a desired steering assist torque is generated. Such a compensation control amount is calculated in various techniques in accordance with the individual purposes.
For example, an electric power steering device proposed in Japanese Patent Application Publication No. JP-A-7-156817 calculates a compensation control amount in accordance with the steering rotation speed in order to acquire a steering assist torque that corresponds to a sharp steering operation, such as a hazard-avoiding emergency steering or the like, while quickening the convergence of the vehicle behavior. In calculating the compensation control amount, the device of Japanese Patent Application Publication No. JP-A-7-156817 multiplies the compensation control amount by a damping coefficient that changes in accordance with the steering torque.
An electric power steering device proposed in Japanese Patent Application Publication No. JP-A-2004-42691 determines a basic damping compensation electric current value in accordance with the steering speed and the vehicle speed, and calculates a damping compensation value from the multiplication product of the basic damping compensation current value and a damping compensation current gain, in order to improve the steering feeling while performing a converging control. In this case, the damping compensation current gain can be switched in accordance with the steering condition.
Furthermore, an electric power steering device proposed in Japanese Patent Application Publication No. JP-A-2002-87309 adopts a construction in which a damping control amount calculated by multiplying the motor rotation speed by a damping control gain Kd, and a steering angular speed feedback control amount calculated by multiplying a control amount calculated on the basis of a deviation of the actual motor rotation speed from a target motor rotation speed computed from the steering torque, by a control gain Go (s), are used as compensation control amounts, in order to improve the follow-up cartelistic regarding the steering wheel operation and the converging characteristic. In this case, the damping control gain Kd is a variable determined by the vehicle speed, the steering torque and the motor rotation speed. Besides, the control gain Go (s) is a variable determined by any one or more of the vehicle speed, the road surface friction coefficient, the road wheel steering angle, and the road wheel steering angular speed.
In the above-described electric power steering devices, the compensation control amount is calculated in accordance with their respective purposes. However, if two or more of the compensation controls as mentioned above are combined, the compensation controls may sometimes interfere with each other, and there is a possibility of failure in achieving a desired steering feeling.
The compensation control amount, which is calculated on the basis of the steering state quantity or the vehicle state quantity, is corrected by multiplying it by a unique control gain that is set for each compensation control. The control gain is determined taking into account events or the like, such as other steering states or other vehicle states. Therefore, in the case where a plurality of compensation controls of different purposes are combined, the individual compensation controls may interfere with one another, the conformation thereof is very difficult. Hence, at the time of development, it is necessary to check the mutual influences of a plurality of compensation controls, which causes increases in the amount of man-hours (the man-hours for the conformation).